The present invention relates to an easily maintained charged-particle accelerator.
In an electron microscope, the electron beam produced by the electron gun may be required to be accelerated with a high voltage, depending upon the purpose of the observation. In addition, the accelerating voltage is required to be kept at a stability of the order of 10.sup.-6. These requirements necessarily make the high voltage power supply bulky.
FIG. 1 schematically shows the circuit configuration of the portion of a conventional, relatively high voltage electron microscope which relates to the acceleration of electron beam. In this figure, surrounded by dot-and-dash line 1 is a high voltage circuit consisting of a voltage-multiplying and rectifying circuit 2, such as a Cockroft-Walton generator. The output from the circuit 2 is supplied to a DC circuit 5 which includes a filtering circuit 4 for filtering out ripple, an output detector circuit (not shown); and an AC power supply 3 for heating a cathode 9. The output from the DC circuit 5 is fed to a control electrode 7 and to the cathode 9 in an electron gun 8 via a balancing resistor r.sub.1 and a bias reactor r.sub.2 through an insulated cable 6. The high voltage circuit 1 and the DC circuit 5 are enclosed in a housing 19, known as a power supply column. This housing is filled with an electrically insulating gas such as Freon (CCl.sub.2 F.sub.2) or sulfur hexafluoride (SF.sub.6). The output from the AC power supply 3 is fed to the cathode 9 in the electron gun 8 through the cable 6. The high voltage from the DC circuit 5 is applied across the terminals of the cathode 9. Voltage-dividing resistors 10 are inserted between the control electrode 7 and the ground. Voltages which are obtained by stepping down the high voltage are applied across a series of accelerating electrodes 11. An accelerating tube is indicated by dot-and-dash line 12. The accelerating electrodes 11 of the accelerating tube 12 and the cathode 9 and the control electrode 7 of the electron gun 8 are enclosed in an evacuated microscope column, indicated by dot-and-dash line 14. The members of the electron gun 8 and the accelerating tube 12 which are outside the microscope column 14 are housed in an electron gun and accelerating tube column 13 that is filled with an insulating gas. A structure of this kind is disclosed in U.S. Pat. No. 3,938,001.
In the prior art structure shown in FIG. 1, the electron gun and accelerating tube column 13 are connected via the insulated cable 6 to the housing 19 that houses the high voltage circuit 1 and the DC circuit 5. As the accelerating voltage is increased, it becomes more difficult to impart flexibility to the cable 6. Where it is a high voltage approximately equal to, or exceeding, 500 KV, the housing 19 and the column 13 must be fabricated as a unit without the use of such a cable. Thus, as described in U.S. Pat. No. 4,160,905, it follows that a power supply column, an electron gun and accelerating tube column, etc. are all enclosed in a single large, sealed housing. Unfortunately, it is difficult to separate this apparatus into several parts for transportation. When a portion of the power supply column is repaired, or when the electron gun or the accelerating tube is cleaned, the insulating gas is required to be fully discharged from the large column before the cover for the column is removed. In this way, the apparatus is uneconomical and cumbersome to maintain.